Last week, we took a look at how Facebook’s removal of third party notifications in March affected player numbers in games like FarmVille in April. However, where it’s apparent that games like FarmVille, which lost 4.4 million players alone, have been affected by the lack of these (formerly) readily available snippets of information, it isn’t just Zynga that has been negatively affected.

Case in point – the second most popular Facebook app, Birthday Cards from RockYou (which isn’t even a game) lost a whopping 5 million of its 39 million users in April, according to Industry Gamers. This makes perfect sense when looked at from the standpoint of the consumer, who must now manually access the app, without the constant reminders available via the ever-present pop-up window in the top left corner of the Facebook interface.

In the case of Birthday Cards, this isn’t the first drop we’ve seen, as in March, 7 million players said goodbye to the virtual greeting card app, whether due to the lack of constant notifications, or some other reasoning.

All of this adds fuel to the fire burning under the backsides of developers, who must continually push to find new ways to interact with their users, either through email or some other means. Fortunately, it doesn’t seem to be that users are simply growing tired of Facebook games (although user fatigue may explain a small portion of the drop on individual games), as the app charts for April saw one particularly interesting example to the contrary – Zynga’s Treasure Isle which launched to the number 5 spot of most popular Facebook apps, with over 25 million monthly active users.

Until May too has passed us by, it will be difficult to do anything but speculate as to how Facebook’s changes will really affect things in the long haul. Presumably, between now and then, we’ll see these developers striving harder to maintain the devotion of their fan base, and it’ll be interesting to see what they come up with.

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